Genetic variation forms the basis of evolution and human diversity. Data from the Human Genome Project originally suggested that any two humans are 99.9% identical in their DNA sequences. The genetic variation that exists between individuals is thought to account for differences in risks to specific diseases as well as differential responses to drugs, infectious agents, toxins, and environmental factors. Until recently, most human genetic variation appeared to be accounted for by single-nucleotide polymorphisms (SNPs), constituting some three million SNPs in each individual genome. Recently, our laboratory (and that of Michael Wigler's) independently discovered the wide-spread existence of copy number gains and losses in the human genome, encompassing hundreds of thousands of basepairs of DNA. Some of the identified variants contain entire genes, and in some cases overlap with known disease loci. In this study, we will use state-of-the-art, cross-platform genomic technologies to better characterize the extent and frequency of this newly discovered type of variation, and its potential to cause or influence susceptibility to disease. This proposal represents the US component of an established international collaboration involving investigators from the US, United Kingdom and Canada. All information generated will be made available in public databases that will have great utility for the clinical and research genetics community.